CMOS imagers are increasingly being used as low cost imaging devices. A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells includes a photogate, photoconductor, or photodiode having an associated a charge accumulation region within a substrate for accumulating photo-generated charge. Each pixel cell may include a transistor for transferring charge from the charge accumulation region to a sensing node, and a transistor, for resetting a sensing node to a predetermined charge level prior to charge transference. The pixel cell may also include a source follower transistor for receiving and amplifying charge from the sensing node and an access transistor for controlling the readout of the cell contents from the source follower transistor.
In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046–2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452–453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe operation of conventional CMOS image sensors, the contents of which are incorporated herein by reference.
A schematic top view of a portion of a semiconductor wafer fragment containing one exemplary CMOS pixel cell is shown in FIG. 1. The CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a charge collection region 21 for collecting charges generated by light incident on the pixel, and a transfer gate 50 for transferring photoelectric charges from the collection region 21 to a sensing node, typically a floating diffusion region 25. The floating diffusion region 25 is electrically connected to the gate 60 of an output source follower transistor. The pixel cell 10 also includes a reset transistor having a gate 40 for resetting the floating diffusion region 25 to a predetermined voltage before sensing a signal; and a row select transistor 80 for outputting a signal from the source follower transistor 60 to an output terminal in response to an address signal.
FIG. 2 is a diagrammatic side sectional view of the pixel cell 10 of FIG. 1 taken along line A–A′. As shown in FIG. 2, the exemplary CMOS pixel cell 10 has a pinned photodiode as the charge collection region 21. Pinned photodiode 21 is termed such since the potential in the photodiode is pinned to a constant value when the photodiode is filly depleted. The pinned photodiode 21 is adjacent to the gate 50 of a transfer transistor. There is a transfer gate/pinned photodiode overlap region 30, where the pinned photodiode 21 and the transfer gate 50 are in close proximity to one another. Additionally, the pinned photodiode 21 has a p-n-p construction comprising a p-type surface layer 24 and an n-type photodiode region 26 within a p-type active layer 20.
In the CMOS pixel cell 10 depicted in FIGS. 1 and 2, electrons are generated by light incident externally and stored in the n-type photodiode region 26. These charges are transferred to the floating diffusion region 25 by the gate 50 of the transfer transistor. The source follower transistor produces an output signal from the transferred charges. A maximum output signal is proportional to the number of electrons extracted from the n-type photodiode region 26.
In conventional CMOS pixel cells having pinned photodiode's, such as pinned photodiode 21, potential barriers may exist near a transfer gate/pinned photodiode overlap region 30, where the transfer gate 50 and the pinned photodiode 21 are in close proximity. FIG. 3 shows a graph representing a potential profile along a cutline L–L′ from the pinned photodiode 21 to the transfer gate 50 in CMOS pixel cell 10. The potential profile shown in FIG. 3 is the potential profile that an electron may encounter as it is transported from the pinned photodiode 21 to the floating diffusion region 25.
An example of a potential barrier 31 which may exist near the transfer gate/pinned photodiode overlap region 30 is shown in FIG. 3. This potential barrier is influenced by several factors, the most important of which are: 1) pinned photodiode 21 donor implant levels, 2) pinned photodiode 21 surface acceptor implant levels, 3) transfer gate 50 threshold voltage adjust implant levels, 4) background p-well concentration, and 5) transfer gate 50 oxide thickness.
The existence of a potential barrier near the overlap region 30 is a problem in CMOS image sensors, particularly for low voltage sensors. The potential barrier results in incomplete charge transfer from the photodiode 21 causing image lag and reduced charge transfer efficiency. Previous methods to reduce this potential barrier have resulted in degraded sub-threshold leakage current for the transfer transistor. It is difficult to optimize both the potential barrier and sub-threshold leakage current for the transfer transistor in CMOS image sensors.
Accordingly, what is desired is a CMOS pixel cell having a reduced potential barrier in an area where a photodiode and a transfer gate structure are in close proximity to one another without increased transfer gate leakage or increased dark current.